The present invention relates to an optical amplification device and an optical amplification method, and particularly relates to an optical amplification device and an optical amplification method that can reduce a noise figure in a wavelength band of an optical signal to be amplified.
In an optical communication network using optical fibers, an optical amplification device is installed in a communication path of an optical signal in order to amplify the intensity of the optical signal that is attenuated by transmission.
For example, an erbium doped fiber amplifier (EDFA) that utilizes an optical fiber whose core is doped with erbium ions is used as a known optical amplification device. In the EDFA, an optical signal input to the optical fiber to which excitation light is supplied is amplified by stimulated emission.
In the EDFA, light generated by spontaneous emission (spontaneous emission light) is added as noise to the optical signal. Therefore, a signal-noise ratio (SNR) of output light deteriorates compared to an SNR of input light. Note that, in the explanation below, the ratio of the SNR of the input light with respect to the SNR of the output light is also referred to as the noise figure. The noise caused by spontaneous emission light cannot be reduced to be less than the quantum limit. Therefore, in optical amplification using the known EDFA, it is not possible in principle to reduce the noise figure to be less than 2 (namely, 3 dB).
Parametric amplification is known as a technology that further reduces the noise figure when amplifying an optical signal. In parametric amplification, an optical signal (hereinafter also referred to as signal light) to be amplified and pump light are input to a non-linear optical element. Then, energy of the pump light is applied to the signal light using four-wave mixing, which is a non-linear optical effect, and thus the signal light is amplified.
A method (hereinafter, also referred to as a PIA-PSA cascade method) is proposed that uses a phase insensitive amplifier (PIA) and a phase sensitive amplifier (PSA) in an optical amplification device that utilizes parametric amplification (for example, refer to R. Tang et al., “Gain Characteristics of a frequency nondegenerate phase-sensitive fiber-optic parametric amplifier with phase self-stabilized input” Optics Express, vol. 13, no. 26, p. 10483, 2005). The PIA and the PSA each include a non-linear optical element and amplify an optical signal by parametric amplification in the non-linear optical element. Here, in the PIA, an amplification gain does not depend on the phase of the input light. Meanwhile, in the PSA, the amplification gain depends on the phase of the input light. The PSA achieves a noise figure of less than 2 (namely, 3 dB), which cannot be achieved by the known EDFA. Therefore, an optical amplification device using the PSA is attracting attention as an optical amplification device used for optical communication.
In the PIA-PSA cascade method, first, the pump light and the signal light are input to the PIA. In the PIA, the signal light is amplified, and at the same time, idler light is generated as wavelength converted light of the signal light.
Next, the pump light, the signal light and the idler light output from the PIA are input to the PSA. The non-linear optical element provided in the PSA is set such that the amplification gain is determined by a relative phase of the pump light, the signal light and the idler light. In the PSA, the signal light is amplified by the amplification gain.